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The Physics of Aero Bicycle Wheels

By Bob Howland

Air, before passing the front wheel, the bike and the rider, is considered to be flowing in a smooth, layered, laminarcondition. In fact, the air is considered to be a flowing fluid. There are three factors of a moving object that affect that laminar air flow: 1) imperfections of the contact surface; 2) the size of the contact surface area; and 3) the shape of the contact surface area.

For the moment ignoring the tire, wheel and the rest of the bike, the air flowing over a round spoke on one side of the spoke “collides” with other air passing around the spoke on the other side. This collision of air molecules is part of what causes the drag turbulence of the spinning spokes of the wheel. These swirling back-flow currents are known as “eddy currents.”

You have seen eddy currents when fast water flows down a river around pilings, rocks, etc. This effect gets worse (squared) the faster you go (see last week’s The Physics of Riding in the Wind).

Eddy Currents and Drag Turbulence

But spokes aren’t the only source of eddy currents on a bike. Pretty much everything that air flows around from front to back creates an eddy current. These constantly changing eddy currents from the spokes, the front tire and wheel, the frame, handlebars and rider, et al, all disrupt that laminar flow and contribute to the final effect: drag turbulence. This is what slows the overall bike and rider.

Focusing again on the wheels, air flows front to back over the lead wheel and tire. The front-to-back air flow has to pass over the tire, rim and most of the spokes of the bike that are in various vertical and diagonal positions as the wheel rotates. So the three factors above are constantly changing for the rotating tire, rim and spokes.

As the air passes over rounded spokes, the air has variable velocities the way air passing over an airplane wing has a faster velocity over the top and a slower velocity under the bottom of the wing. But, unlike a fixed wing on an aircraft, the tire, wheel and spokes are constantly moving, disrupting the laminar air flow. Instead, the tire, wheel and spokes are producing constantly changing air flow velocities.

Aero Wheels “Smooth” the Air Flow

Minimizing those three factors mentioned previously — 1) imperfections of the contact surface; 2) the size of the contact surface area; and 3) the shape of the contact surface area — partially restores the laminar flow of air over the wheels.

Aero wheels do this by smoothing out the eddy currents, allowing the wheels to effectively “knife” more quietly through the air. They create less drag turbulence. Ideal aero wheels get better at this as they spin faster.

Most manufacturers will tell you that the aero properties do not really come into play until a certain speed threshold has been passed — typically at the very least 20 mph (32 kph). There is actually some pretty serious physics here tied in with torque, rotational inertia, centripetal force, drag coefficients, etc. — but That’s too complex for this discussion.

Stability a Byproduct of Aero Wheels

Modern aero wheels help the laminar flow stay along the spinning wheel and spokes. As long as the wheel is going straight, the laminar flow of the air “fluid” produces a “pressure” on each side of the spinning wheel that helps it resist turning.

Further, even moderate steering to the left or right — “yaw” — is self-correcting. That is, the bike tries to straighten out on its own.

The bottom line is this: Based on what I already knew and discovered from new research, the exact reasons a bike has rolling stability are very complex and are not completely understood.

The good news: 1) rolling bikes are stable; 2) rolling bikes are self- correcting despite small steering changes, and 3) aero wheels are even more stable than standard wheels due to the laminar “pressure” of the air fluid moving on each side of the rolling wheel.

Aero wheels with deep-dish rims help create better laminar flow, thus helping the rolling wheel stay stable and straight on its course — yet more difficult than non-aero wheels to steer away from that true course. But an aero wheel with traditional spokes cuts down on that steering resistance compared to 3-wide-spoked aero wheels and thus may be the best of both worlds.

Wind Not the Friend of Aero Wheels

The trade-off with aero wheels is that the “knife-slicing” surface when going straight ahead turns into a liability in the presence of any sort of crosswind. Crosswinds become difficult in the first place due to the surface area you as a rider present to the wind. Aero wheels increase that surface area in crosswinds.

If the scenario is a diagonal headwind, there is little you can do except “get small” on the bike. Research shows it is best to reduce side-side surface area. Bring in your arms, elbows and legs, and get low over the bars. Finally, aero wheels or not, a direct side wind may just push you all over the road. It will hit your body at a perpendicular angle and there is very little you can do but slow down and lean into the wind to stay upright.

All of this is to say that the effects of wind are compounded with deep-dish aero wheels. Those wheels in any serious crosswind can act like uncontrollable “sails.” That means your bars may end up moving in your hands as if possessed, making illogical, erratic tugs left and right due to the momentum and push force of the crosswind.